Dry pipe automatic fire protection sprinkler systems are typically employed for the purpose of providing automatic sprinkler protection in unheated occupancies and structures that may be exposed to freezing temperatures. The dry pipe sprinkler system is connected to a public or private water main providing a reliable supply of water, and typically includes an indicating type of water flow valve, e.g. a water gong or other alarm flow valve, a fire department connection and a dry pipe valve. The dry pipe system is used primarily in unheated warehouses and the like where water-filled pipes cannot be used, so the dry pipe valve must be protected against freezing by locating it in a heated portion of the structure, e.g. in the warehouse office or in a heated enclosure provided for the purpose, to protect the dry pipe valve from freezing.
The sprinkler portion of a dry pipe sprinkler system has an arrangement of piping similar to a wet pipe sprinkler system. However, rather than water, the dry pipe sprinkler system contains air or nitrogen under pressure above the dry pipe valve. The air pressure restrains the water in the supply main at the dry pipe valve by holding the valve in closed position until one or more sprinklers open, e.g., in the presence of fire. The loss of air pressure allows the dry pipe valve to open, permitting flow of water through the valve into the arrangement of piping and on to the open sprinkler at the location of a fire.
Many dry pipe valves are of the differential-type, single clapper construction. These center differential pressure valves are designed with a dry system seat and a water supply seat concentrically located with their axes at an equal distance from the center of the clapper hinge pin. As seen from the following equation, the differential ratio is the relationship of the air seat area divided by the water seat area:DF=(AD/WD)2*L2/L1AD=√{square root over (DF*WD2)}where:                AD=system (air) valve seat mean diameter        WD=supply (water) seat mean diameter        DF=differential, i.e., the ratio between the system water pressure and system air pressure (where 5.5 to 6.0 is the industry standard)        L1=distance between the center of the hinge or pivot and the center of air pressure (i.e., the air valve seat axis)        L2=distance between the center of the hinge or pivot and the center of water pressure (i.e., the water valve seat axis)        
By way of example, for a 6-inch single clapper, differential-type dry pipe valve, where:WD=6; DF=5.5; L1=L2AD=√{square root over (DF*WD2)}=14 inches
In the case of a typical ratio of 5.5 (the industry standard), a 6-inch diameter water supply thus requires a 14-inch diameter air valve seat. This valve design is very reliable and made from relatively few parts; however, this relationship also results in a valve that is relatively large and heavy, and therefore difficult to install. An alternative design for achieving a relatively lower weight is a mechanical latching dry valve. This type of dry pipe valve is relatively smaller in size, but it requires more components, and it is often more difficult to maintain because of the relatively greater number of parts located within its auxiliary chamber.